AbstractIn a very smal1 silver speck, (1) its potential shifts discontinuously in a positive or negative direction by the addition or subtraction of an atom to or from the speck and (2) its potential is separated into two discrete levels; the lowest unoccupied and the highest occupied level.By taking into consideration (1). or (2), the rates of growth and decay of the speck in the developer were calculated. It was shown that the discrimination between exposed and unexposed grains during development is due to the difference in the reduction rates, and even though small specks undergo a temporary oxidation by the developer whose redox potential is more negative than that of massive silver, they are undiscriminatively at the same rate as other Ag+ ions on the grain.